The proportionality of products is not a revelation that we pretend to offer to the world, or a novelty that we bring into science, any more than the division of labor was an unheard-of thing when Adam Smith explained its marvels. The proportionality of products is, as we might prove easily by innumerable quotations, a common idea running through the works on political economy, but to which no one as yet has dreamed of attributing its rightful importance: and this is the task which we undertake today. We feel bound, for the rest, to make this declaration in order to reassure the reader concerning our pretensions to originality, and to satisfy those minds whose timidity leads them to look with little favor upon new ideas. The economists seem always to have understood by the measure of value only a standard, a sort of original unit, existing by itself, and applicable to all sorts of merchandise, as the yard is applicable to all lengths. Consequently, many have thought that such a standard is furnished by the precious metals. But the theory of money has proved that, far from being the measure of values, specie is only their arithmetic, and a conventional arithmetic at that. Gold and silver are to value what the thermometer is to heat. The thermometer, with its arbitrarily graduated scale, indicates clearly when there is a loss or an increase of heat: but what the laws of heat-equilibrium are; what is its proportion in various bodies; what amount is necessary to cause a rise of ten, fifteen, or twenty degrees in the thermometer,--the thermometer does not tell us; it is not certain even that the degrees of the scale, equal to each other, correspond to equal additions of heat. The idea that has been entertained hitherto of the measure of value, then, is inexact; the object of our inquiry is not the standard of value, as has been said so often and so foolishly, but the law which regulates the proportions of the various products to the social wealth; for upon the knowledge of this law depends the rise and fall of prices in so far as it is normal and legitimate. In a word, as we understand by the measure of celestial bodies the relation resulting from the comparison of these bodies with each other, so, by the measure of values, we must understand the relation which results from their comparison. Now, I say that this relation has its law, and this comparison its principle.